The present invention relates in general to a demodulation system for a multi-level, multi-phase, superposition-modulated carrier wave, and more particularly to a demodulation system of a simple construction for demodulating a multi-level, multi-phase, superposition-modulated carrier wave that is formed by synthesizing two phase-modulated waves mutually synchronized.
Now that digital transmission has been in practical use, efforts have been made recently to enhance the transmission capacity of a carrier wave and the efficiency of the use of frequency bands. One of the proposals made as a result of such efforts is the multi-level, multi-phase, superposition-modulated signal transmission system in which a first and a second modulated carrier waves are synthesized under synchronized state to constitute a composite carrier wave for transmission. An ordinary multi-level, multi-phase, superposition-demodulator apparatus for such transmission system includes the so-called re-modulation type demodulator synchronized with the incoming transmitted carrier wave, and a vector subtractor coupled to the demodulator.
The demodulator is adopted to demodulate at its first demodulator section the first modulated carrier wave contained in the modulated carrier wave. Also, the second modulated carrier wave is obtained by deriving the remodulated first modulated carrier wave from the first modulator section and subjecting it to vector subtraction. This second modulated carrier wave is demodulated at the second demodulator section similar to the first demodulator circuit.
This demodulation system in the prior art has a disadvantage that it necessitates the first and second demodulator circuits and the vector subtractor circuit to obtain the second demodulated carrier waves, complicating the circuit construction. In addition, it is very difficult to obtain a stable modulated carrier wave from a vector subtractor circuit. Furthermore, in the first demodulator circuit it is necessary to obtain a first modulated carrier wave besides the first demodulated signal, so that a re-modulation type phase synchronizer circuit that is simple in construction must be employed. Accordingly, there is a restriction to the circuit construction.